How much is 2 years of having the leagues leading goalscorer worth to Everton?
..it's a good point and a very difficult one to answer. Leading scorers aren't everything (as we found out with Lineker) so much depends on our skill in the transfer market. We are in a business and a financial balance needs to be analysed. Every player has a value and the deciding factor will surely be whether a club are prepared to meet Everton's asking price.
The other point is the worth of keeping players who want to be elsewhere. Keeping Stones an extra season was applauded at the time but didn't really work in anybody's favour because the players form dipped and he became a disruptive figure.